The invention relates to an electro-mechanical grating device, and more specifically, to an electro-mechanical grating device having a continuously controllable diffraction efficiency.
Electro-mechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing, and printing. Optical beam processing for printing with deformable mirrors is well known as is a device for optical beam modulation using cantilever mechanical beams. Other applications of electro-mechanical spatial light modulators include wavelength division multiplexing and spectrometers.
Electro-mechanical gratings are also well documented in the patent literature; see U.S. Pat. No. 4,011,009, issued Mar. 8, 1977 to Lama et al., entitled xe2x80x9cReflection Diffraction Grating Having a Controllable Blaze Angle,xe2x80x9d and U.S. Pat. No. 5,115,344, issued May 19, 1992 to J. E. Jaskie, entitled xe2x80x9cTunable Diffraction Grating.xe2x80x9d More recently, Bloom et al. described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV); see U.S. Pat. No. 5,311,360, issued May 10, 1994, entitled xe2x80x9cMethod and Apparatus for Modulating a Light Beam.xe2x80x9d This device was later described by Bloom et al. with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces; see U.S. Pat. No. 5,459,610, issued Oct. 17, 1995, entitled xe2x80x9cDeformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate.xe2x80x9d
In Bloom ""610, a device is described in which the ribbons are deflected in a continuous range of heights above the substrate by varying the applied voltage. The space between the ribbons and the ground plane in this device must be relatively large in order to have continuous control of the diffraction efficiency allowable by the device. However, if the ribbons are actuated and thereby contact the surface of the substrate, either by error or due to charge accumulation in the dielectric ribbon material, the mechanical strain of the ribbon can exceed the ribbon material""s critical strain and cause mechanical failure (i.e., breakage, cracking, or wear of the ribbons). Consequently, what is needed is an electro-mechanical grating device that can be driven to provide continuous control of the diffraction efficiency and that has less mechanical failure of the ribbon elements.
The aforementioned need is met by providing an electro-mechanical grating device including: a base having a surface; a bottom conductive layer provided above said base; a spacer layer is provided and a longitudinal channel is formed in the spacer layer, wherein the spacer layer defines an upper surface and the channel having a first and a second opposing side wall and a bottom; a plurality of spaced apart ribbon elements disposed parallel to each other and spanning the channel, the ribbon elements are fixed to the upper surface of the spacer layer on each side of the channel and each of the ribbon elements is provided with a conductive layer; a mechanical stop provided between the bottom conductive layer and the bottom of the channel wherein the mechanical stop forms a rigid barrier that is separated from a lower ribbon surface of the ribbon elements by a distance h0, and that causes actual deflection distance of the ribbon elements to be limited to h0 upon application of a pull-down voltage, thus reducing breakage of the ribbon elements, and wherein dmax greater than h0 greater than xcex/4, where dmax is the ribbon deflection at which pull down occurs and xcex is a wavelength of light to be deflected by the electro-mechanical grating device.
Furthermore, the above need is accomplished by a second embodiment of the invention. The second embodiment is an electro-mechanical grating device, which includes: a base having a surface; a bottom conductive layer provided above said base; a spacer layer is provided and a longitudinal channel is formed in the spacer layer, the spacer layer defines an upper surface and the channel having a first and a second opposing side wall and a bottom; a plurality of spaced apart ribbon elements disposed parallel to each other and spanning the channel, said ribbon elements are fixed to the upper surface of the spacer layer on each side of the channel and each of the ribbon elements are provided with a conductive layer, the ribbon elements have a thickness tr; a protective layer provided between the bottom conductive layer and the spacer layer, the protective layer has at least a thickness ts, wherein             t      s        =                  1        3            ⁢              (                                            t              r                                      ϵ              r                                +                      λ            4                          )              ,
xcex5r is a dielectric constant of the ribbon elements, and xcex is a wavelength of light to be deflected by the electro-mechanical grating device; and a plurality of standoffs, patterned within the width of the channel, located atop the protective layer, and having a height equal to the thickness of the protective layer such that a mechanical stop is constructed.
Another embodiment is an electro-mechanical grating device, which includes: a bottom conductive layer provided above the base; a spacer layer is provided and a longitudinal channel is formed in the spacer layer, the spacer layer defines an upper surface and the channel having a first and a second opposing side wall and a bottom; a plurality of spaced apart ribbon elements disposed parallel to each other and spanning the channel, the ribbon elements are fixed to the upper surface of the spacer layer on each side of the channel and each ribbon element is provided with a conductive layer, the ribbon elements have a thickness tr; a protective layer provided between the bottom conductive layer and the spacer layer, the protective layer has at least a thickness ts wherein             t      s        =                  1        3            ⁢              (                                            t              r                                      ϵ              r                                +                      λ            4                          )              ,
and the protective layer has a plurality of standoffs patterned within the width of the channel such that the standoffs have a height less than the thickness of the protective layer, while residing atop the protective layer to form a rigid barrier for the ribbon elements, once they are actuated.
It is advantageous that the electro-mechanical grating device produces continuous control of the diffraction efficiency from the minimum to the maximum efficiencies. A mechanical stop consisting of at least one dielectric layer that forms a rigid mechanical barrier is provided to prevent over-actuation of the ribbon elements, thereby, eliminating the risk of ribbon element breakage, ribbon element cracking, or severe ribbon element wear. The rigid mechanical barrier consists of one or more layers of dielectric material, situated between the ground plane and the air gap, that is either continuous or patterned to form a stand-off structure fabricated on top of the ground plane.